The stator core of an electric generator is made up of a large number of laminated sheets. These laminated sheets are thin iron sheets with oriented crystals. They are insulated from one another to reduce losses due to eddy (Foucault) currents between the laminations. The assembly of laminations may be mechanically reinforced through a plurality of wedges through the stator yoke as well as rings and pressure plates.
During assembly and also during operation, faults of the insulation between the laminated sheets may occur as a consequence of thermal and magnetic stresses and of mechanical strains and vibrations. These faults short-circuit sheets of the stator. They may result in significant eddy currents circulating between the faulted sheets. The losses due to such eddy currents may result in iron melting and even in thermal failure of the electrical insulation of adjacent stator bars.
There are two established methods of test known as high-energy and low-energy tests. The present disclosure focuses on high-energy tests.
A high-energy test requires a magnetic flux density of about 1.0-1.5 Tesla to be induced in the stator core. The flux density alternates with time in a way similar to the flux density in service. Due to eddy currents sheets with short-circuits will exhibit an increase in temperature which is significantly higher than the average temperature of the stator core. Local overheating is then detected by means of temperature measurements. To that end, infrared cameras may be used.
The high-energy method of testing stator cores has got a number of disadvantages. It requires a high-power supply and a high-power excitation winding. The excitation winding typically consists of several windings of a cable and is adequately dimensioned so the high-power supply can drive sufficient alternating current though the excitation winding. To achieve a magnetic flux density of 1.5 Tesla through the stator core, the high-power supply must provide significant voltage and current.
The currents would typically be in the range of several kA and the voltages in the range of several kV. Consequently, the amount of inductive reactive power required for the test is in the range of several MVAr. The excitation of a 330 MW turbogenerator or of a 50 MW hydro generator would typically require a high-power supply in the form of 4 MVA, 6.3 kV transformer. On-site, neither the power grid nor any other source may be able to supply 4 MVA of predominantly reactive power to a 4 MVA, 6.3 kA transformer.
The above inductive current can, at least in part, be compensated through a capacitor. That capacitor would be connected in parallel to the excitation winding. Especially when testing large electric generators, the windings of the excitation winding may be arranged symmetrically around the stator core. The symmetrical arrangement of the excitation winding yields a more uniform distribution of the magnetic flux density through the core.
The voltages of several kV applied in the high-energy test create a hazard to any personnel in the vicinity of the test. This applies both to the excitation winding and also to any transformer feeding that winding. Consequently, precautions for high-voltage tests such as job safety assessment, fences with interlocks around any high-voltage equipment, switches for emergency de-energization etc apply. All of those precautions make the procedure more onerous and add to the cost of high-energy testing.
Another problem arises due to the non-linear saturation curve of stator iron. The relationship between the magnetic flux density B in the stator core and the excitation current I through a winding with N loops can be described asB∝N·I 
This relationship is, however, valid only in the linear regime. As the current I through the excitation winding increases, the stator core made of iron laminations saturates. The relationship between the magnetic flux density B and the excitation current I then becomes non-linear. Due to saturation the current I through the excitation winding will increase faster than linear with the magnetic flux density B for B≧1.3 Tesla. It can actually become practically unattainable to supply the reactive current because no adequate source of inductive current is available.
Another approach may make use of a power electronic converter to supply the excitation winding. The reactive power required for the excitation winding can at least in part be provided by a circuit for energy storage integrated in the power electronic converter. An advantage of this solution is that all load cases up to maximum reactive power are covered. A disadvantage of this solution is that the power electronic converter needs be designed for maximum load. This, in turn, adds to the cost of the converter. In addition, the power electronics components inside the converter must be designed to withstand voltages of several kV. Due to the nature of power electronics components such as thyristors and insulated-gate bipolar transistors the requirement of high voltage withstand be difficult to meet.
The approach set out in EP2541751 partially overcomes these issues by providing a plurality of excitation modules each with an excitation winding. The excitation windings are arranged around the stator core and every excitation winding provides a part of the overall excitation. Consequently, the voltage over each excitation winding becomes only a part of the voltage that would exist if there was only one excitation winding. In other words, the aforementioned high-voltage hazard is mitigated. The approach as set out in EP2541751 still falls short of solving the high-current issue. As mentioned above, it may become practically unattainable to supply the full excitation current when the magnetic flux density through the stator core goes into saturation.
The present disclosure is oriented towards providing the aforementioned needs and towards overcoming the aforementioned difficulties.